Our social customs vary between cultures. The reasons for this are complex and varied. And though there were probably legitimate historical reasons behind different cultural norms they can still catch us off guard. They are not so much odd as simply unexpected. When traveling it is best not to offend local residents.
Small talk is fine, but the subject of the small talk can be an issue. Apparently some Europeans feel uncomfortable discussing their occupation, or at least being asked about it. Many people in the USA consider discussing income to be awkward and boastful; maybe this is because they are trying to believe in egalitarian ideals, and different income shows the inequality.
Politics is often a bad topic of discussion, at least if it is the other person’s politics. The best way out may be to say that you don’t understand (local) politics, or that you are not sure that anybody does.
Tipping at a restaurant or for any service varies greatly between countries. In the USA it is expected. In much of Asia it is felt to be an insult. You will have to figure this out for each country (Google for information).
We are all in the unconscious habit of keeping others at a slight distance, but we only really notice this when that space is intruded upon. Southern Europeans and South Americans have minimal personal space; the USA, Asia and some Europeans tend to put a little space between themselves and others. This varies with gender and familiarity- opposite gender strangers being expected to stand further away. As we are the visitors overseas it is probably best to let the locals determine appropriate space. You are safe if not for pickpockets.
Dressing in a sloppy manner is offensive in some places. Dressing neatly is unlikely to offend anybody.
In Asian counties and the Middle East, and any Buddhist or Hindu country, it is extremely offensive to show the sole of you shoes or feet. Avoid crossing your legs this way.
Blowing your Nose.
It is offensive in many places to blow you nose in public, or at the dinner table. Use the bathroom for this.
In the USA people get into the back seat of a taxi. In Australia we usually use the front seat.
This varies between countries. Read up upon local customs for accepting and offering gifts.